This project is designed to determine the mechanism(s) by which the early bovine embryo causes maintenance of the corpus luteum. Detailed hormone analyses are being made in three groups of aninals as follows: (a) pregnant, (b) non-pregnant cycling, and (c) non-pregnant after insemination. Animals at both Day 17 and 18 are being studied. The major thrust of the investigation is to determine if the corpus luteum is maintained in early pregnancy by a luteotrophic mechanism involving the conceptus or by a mechanism involving inhibition of the uterine luteolytic mechanism. Animals are subjected to surgery at Day 17 or 18 and the following samples collected: (a) jugular blood, (b) ovarian arterial blood, (c) uterine venous blood, (d) ovarian venous blood, (e) corpora lutea are removed and (f) hysterectomies are performed and the embryos flushed from the uterus. Estrogens, testosterone, progesterone, arachidonic acid, PGF2 alpha and LH are being measured in blood samples; the corpora lutea are tested for their ability to synthesize progesterone, PFG2 alpha and other steroids; the embryos are being tested for LH-like activity. Results obtained to date favor the idea that the early embryo produces a luteotrophic substance. No clear cut evidence in favor of the concept that a uterine luteolytic mechanism is inactivated has been obtained.